Nuclear density gauges are often used to measure liquids (the term liquid, when used in this application for patent, includes slurries) in a vessel at places such as oil refineries. As illustrated in FIG. 1, a nuclear density gauge often includes a source well that extends into the vessel from the top thereof, so that a nuclear source can be installed into the source well and then radiation from the source can be detected, e.g. with a scintillation detector, from outside of the vessel to determine factors such as level and density of the liquid in the vessel. When radiation leaves the source, the amount reaching the detector aligned with the source decreases as the distance between them increases, even if there is only a vacuum between them. When passing through a liquid in a vessel, the mass of the liquid absorbs some of the radiation. In addition to the liquid, if the source, the detector, or both, are located outside the vessel, the radiation must also pass through at least one of the vessel walls. Since the absorption of the radiation by the walls is constant, and the distance between a source and a detector is constant, the amount of radiation reaching the detector is indicative of the presence of liquid between the source and detector, and of the density of the liquid it passes through. As the density of the liquid changes, the amount of radiation reaching the detector changes. The greater the density of the liquid, the less radiation reaches the detector. The detector typically provides density measurement in the form of a current output.
It should also be noted, that to keep a detector cool, and make it easier to maintain, detectors are almost always located outside the vessel. The source, however, can be durably packaged, and more easily placed inside the vessel. A source well enables the source to be placed within the vessel, hanging on the end of a cable, and separated from the liquid. This allows maintenance, calibration and testing of the source on an appropriate schedule by withdrawing the source from the well using the cable.
In some installations, multiple sources are placed in the well at varying heights, to allow measurement of level and density at multiple points within the vessel for better process control. Each source is typically hung in the well on its own cable so that the sources can be separately withdrawn for maintenance, and their heights can be separately controlled.